Telephone systems commonly employ copper wires as conductors for telephone circuits. Copper wires are subject to faulting, resulting in a degradation or loss of telephone signals and thus of telephone service.
Two of the most common faults are resistive faults and open faults. A pair of conductors with a resistive fault will typically have a continuity of either high or low impedance between the two conductors or between one of the conductors and ground. A pair of conductors that is free of faults does not exhibit continuity between the conductors or between one conductor and ground. A resistive fault is often caused by water entering the cable containing the conductors or by physical damage such as pinching or crushing the cable.
An open fault is present when a pair of conductors has lost continuity on either one or both sides of the pair. For example, an open fault can be caused by a cut cable, wherein the cut would break the continuity of one or both of the conductors in a pair. Another example of an open fault is where a cable splice failed to provide electrical conductivity between sections of a cable.
It is often the case that cables have a combination of faults. For example, a cable that has been cut may have an open fault but may also have conductivity to ground, and will therefore also have a resistive fault.
Due to the way in which faults are typically measured, cables that have combinations of both open and resistive faults present special problems for field technicians attempting to analyze and locate the faults. Open faults are normally analyzed by measuring the capacitance of one side of the pairs to ground, or by measuring the mutual capacitance of the pair. Knowing the relationship of capacitance per unit length, it is then possible to determine the distance to the open fault. Once the distance to the open fault is known, the field technician can take corrective action.
Capacitance may be measured by one of several techniques. A typical technique is to charge the line to a known voltage and/or discharge the line from a known voltage and measure the current required. The capacitance can then be determined from the charging or discharging current. However, if the line being measured has a resistive fault in addition to an open fault, a substantial error may be present due to the current or charge which will leak through the resistive path and not remain on the line for measurement purposes.
This problem is analogous to filling up a bucket with water to measure its volume. If the bucket has a hole in it, there will be an error in the measurement. This is because while the amount of water in the bucket is being measured, water is leaking out through the hole and is thus unaccounted for. In this analogy, the bucket is the cable to be measured, its volume is the capacitance of the line to be measured, and the hole is the resistive fault.
Prior art test sets typically check for resistive faults by placing a current source between two points (e.g. between the two conductors in a pair or between a pair conductor and ground) and measuring the resultant voltage. The resistance between the two points can be calculated using Ohm's law. If the measured resistance is high enough, then it will leak very little charge and have little effect on the capacitance measurement. On the other hand, if the resistance is low, the stored charge will quickly bleed off and the error on the capacitance measurement will be large.
The prior art approach suffers from several disadvantages. One disadvantage is that current sources normally have a limited operating range and accordingly have a limited measurement capability at high values of resistance. Another disadvantage is that the value of a resistive fault may depend on the amount of voltage that is applied across the fault. In fact, a resistive fault can even break down when subjected to a voltage.
Another disadvantage, and one which is important, is that simply measuring the resistance of a fault does not determine the amount of error in the capacitance measurement. This is because the amount of capacitance measurement error depends on several factors in addition to the value of the resistance. One such factor is the length of the line or conductor. If the line is long, it can store a large amount of charge, that is its capacitance is large. For a given resistance, the line will take a long time to drain the charge. However, if the line is very short, that is its capacitance is small, any resistive leakage will quickly drain all of the charge. Thus, capacitance and resistance are interrelated to each other. This interrelation may be expressed in terms of voltage: V(t)=V.sub.0 e.sup.-t/Rc, where V(t) is the voltage across the circuit at time t, V.sub.0 is the initial voltage, R is the resistance and C is the capacitance of the circuit.
Another such factor why simply measuring resistance will not accurately determine the error in a capacitance measurement is the amount of delay time between charging the line and then discharging the line for the capacitance measurement. Between the time that the charging cycle ends (for example, no more charge is being applied to the line) and the time that the capacitance measurement begins is a finite delay time. During this delay, some charge leaks through the resistive fault, producing error. Line length and the value of the fault resistance all affect the amount of error caused by the delay.
Thus, it is desirable to determine the error produced by a resistive fault in an open fault measurement, given all of the variables of impedance of the fault, line length and idle time between measurements.